As is well known, television broadcasting systems are not unified all over the world. There are basically three television systems depending on the transmission format of the color signal. Therefore, there has arisen a demand for a system which can be used universally throughout all the regions.
In order to meet such a demand, there has been developed a technique in which an NTSC system can be used in an area of the PAL system, and in which the PAL system can be used in an area of the NTSC system. U.S. Patent No. 4,617,597 discloses such a technique in which different broadcasting systems can be used in different areas using the NTSC or PAL systems.
As is well known, the NTSC system and the PAL system are significantly different with respect to their number of scanning lines, that of the former being 525, and that of the latter being 625.
When the NTSC system is converted to the PAL system, the number of scanning lines is increased from 525 to 625, while, when the PAL system is converted to the NTSC system, the number of scanning lines is decreased from 625 to 525. However, in such a case, the picture becomes unstable, and the reproducing precision is deteriorated.
That is, when the number of scanning lines is increased from 525 to 625, one line is stored for every 5 lines, and the scanning is carried out in an overlapping manner. Thus, if 6-line scanning is performed, a scanning of 525 lines becomes equivalent to a scanning of 625 lines.
Further, when the number of scanning lines is decreased from 625 lines to 525 lines, one line is deleted during scanning of every 6 lines, so that actually 525 lines should be scanned when a scanning of 625 lines is performed.
However, in the above cases of conversion, either the same scanning line is scanned a second time, or scanning lines are deleted, and therefore, the picture is reproduced in imperfect form and becomes unstable.
Particularly, in a large scale picture tube, such instability of the picture is a serious problem.